Light emitting diodes (“LEDs”) are technologically and economically advantageous solid state light sources. LEDs are capable of reliably providing light with high brightness, hence in the past decades they have come to play a critical role in numerous applications, including flat-panel displays, traffic lights, and optical communications. When driven by a current, electrons and holes are injected into the junction region, where they recombine, releasing their energy by emitting photons. The efficiency of LEDs has important consequences on such applications. Conventional surface-emitting LEDs with high internal efficiencies suffer from relatively low external quantum efficiency. Quantum Dot LEDs, for example, exhibit external quantum efficiency of about 2-5%. These low external quantum efficiencies are independent of LED configuration, which may be bottom emitting, top emitting and/or inverted configuration.
External quantum efficiency is the ratio of the number of photons emitted from an LED to the number of electrons injected into the LED. External quantum efficiency quantifies how efficiently an LED device coverts electrons to photons and allows them to escape in the form of visible light, or in some cases, infrared light. One way in which improved external quantum efficiency can be observed in by an increase of optical output by a positive aging effect.
External quantum efficiency is a function of injection efficiency, internal quantum efficiency and extraction efficiency. Internal quantum efficiency is the proportion of all electron-hole recombination in the active region that produces photons, while extraction efficiency is the proportion of photons generated in the active region that escape from the device. Certain approaches have been attempted to increase external quantum efficiency such as roughening one LED surface and applying a back mirror. This causes reflection of light that is normally internally reflected within the LED, to escape the LED increasing the light intensity, thus improving the extraction efficiency, and the external quantum efficiency.
Methods to maximize internal quantum efficiency and injection efficiency and external quantum efficiency are relatively well known in the art. Internal quantum efficiency and injection efficiency can be maximized by material selection for the appropriate bandgap, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels for organic layers, electron affinity, ionization potential, and work function for an inorganic layer during the construction of a device. Some methods have been used to increase external quantum efficiency such as using back mirrors, etched waveguides, reflective substrates or reflective intermediate layers within a LED, with limited success. Notwithstanding, to date no one has examined the effects of the fabrication process on an LED's external quantum efficiency.
It is noted that certain improvements to an LED's external quantum efficiency has been made through the use of Quantum dot light emitting diodes. Quantum dot light emitting diodes (QD-LEDs) have been developed for display and lighting sources. Inorganic quantum dot light emitters have a few advantages over organic light emitting diodes (OLEDs) and other light-emitting diodes, more important of which is excellent color purity. Quantum dots (QDs) are semiconductor nanocrystallites whose radii are smaller than the bulk exciton Bohr radius. Quantum confinement of electrons and holes in all three dimensions leads to an increase in the effective band gap of the QDs with decreasing crystallite size, where the optical absorption and emission of quantum dots shift to higher energies (blue shift) as the size of the dots decreases. For example, a CdSe QD can emit light in any monochromatic visible color depending only on the size of the QD and can be used to form QD-LEDs arrays that emit white light. The emission color of quantum dot can also be tuned by its composition. For example, ZnxCd1-xS emits blue, ZnxCd1-xSySe1-y emits green, and ZnxCd1-xSe emits red. Suitable quantum dots are taught in US 2012/0138894 A1, which is incorporated herein in its entirety. It is appreciated that use of quantum dots improves devices external quantum efficiency. Nanoparticles are also known for improving the characteristics of devices. In particular, the incorporation of zinc oxide nanoparticles into LED devices further improves the efficiency and lifetime of LED devices
Notwithstanding, there still remains an unmet need to improve the external quantum efficiency of LEDs in the fabrication process of the components of the LED, such as the anode, the cathode, or quantum dots, to name a few. There further remains an unmet need to provide a method to assist in improving an aged LED's external quantum efficiency.